Page 377 - Biaxial Multiaxial Fatigue and Fracture

BiaxiayMultiaxial Fatigue and Fracture
Andrea Carpinten, Manuel de Freitas and Andrea Spagnoli (Eds.)
0 Elsevier Science Ltd. and ESIS. All rights reserved. 361

THE ENVIRONMENT EFTECT ON FATIGUE CRACK GROWTH RATES
IN 7049 ALUMINIUM ALLOY AT DIFFERENT LOAD RATIOS

Manuel FONTE’, Stefanie STANZL-TSCHECG’, Bemd HOLPER’,
Elmar TSCHEGG3 and Asuri VASLJDEVAN4
’ Nautical School, 2780-572, Portugal.
’University of Agricultural Sciences, Wien, Austria.
Technical University of Wien, Austria.
Naval Research Laboratory, Arlington, VA 2221 7, USA.

ABSTRACT

The influence of environment and microstructure is investigated on a high strength 7049
aluminium alloy sheet cold rolled and heat-treated. This aluminium alloy was artificially aged
to underaged (UA) and overaged (OA) conditions, resulting in approximately the same yield
strength, but different mode of slip deformation. The UA alloy deforms by planar slip while
the OA alloy by wavy slip. Crack growth measurements were performed at constant load
ratios between -1 and 0.8 in ambient air and vacuum. The influence of load ratio is discussed
in terms of slip deformation mechanisms, microstructure and environmental effects using the
two intrinsic parameters, dK and K-. The two parameters lead to two intrinsic thresholds
that must be simultaneously exceeded for a fatigue crack growth. Mechanisms of near-
threshold crack growth are briefly discussed for several concurrent processes involving
environmentally assisted cracking with intrinsic microstructural effects.

KEYWORDS
Fatigue crack growth, near-threshold fatigue, environment, microstructure, AK,
K,,,
slip mode, load ratio effects, 7049 and 7075 aluminium alloys.

INTRODUCTION

Microstructure and environment strongly influence the fatigue crack growth resistance of
aluminium alloys and have been investigated for the last two decades [I-91. However, the
nature of the underlying interactions between microstructure and environment is still not
clearly understood. Synergetic effects of environment and loading make the understanding of
the underlying mechanisms between microstructure and environment difficult.
Since the mid- 1970s, increasing demands for fail-safe designs and damage-tolerant
constraints have given importance to the fracture toughness and fatigue crack growth
resistance properties [10,1 I]. The significance of grain boundaries (GB) precipitations on